This invention relates to a method of improving the efficiency of removing the volatile components, such as unreacted monomers, solvents, low molecular weight components, water and dissolved gases, from the thermoplastic plastics, e.g. polystyrene, polymethyl methacrylate, styreneacrylonitrile copolymer, etc. The method consists of two processes: a flashing process, the polymer solutions are first devolatilized at atmospheric pressure; a process for extruding the polymers, which is the so-called extrusion devolatilizer, the polymers are devolatilized in a single- or twin- or multi- screw extruder (hereafter referred to as an "extruder"), wherein a plurality of devolatilizing (covering the sense of degassing) sections are connected in series with one another. In both devolatilizing processes the accoustic treatments are applied to increase the devolatilizing efficiencies.
Extruders capable of extremely efficient devolatilizing are required for use in the preparation of bulk thermoplastic plastics, both for legal reasons and for the reasons of economy. Hence, the input thermoplastic solution which is discharged from the polymerization reactor must be devolatilized from a volatile content of up to about 90% by weight to a content of 50.about.1000 ppm, depending on the polarities and boiling points of monomers and low molecular weight components which decide the ease of removal of monomers and low molecular weight components. As is often the case, of the volatile components, monomers and low molecular weight components are more difficult to be removed and contribute to the large part of the residual volatile components in the final extruded bulk products.
By referring to a recent article, Foster et al., "Bubble Growth Controlled Devolatilization in Twin-Screw Extruders", there are three sections of devolatilization that are defined. The first section of devolatilization is a flashing process in which the preheated polymer solution at the temperatures well above the normal boling points of the volatile components is fed into a vessel with an atmospheric pressure outlet. Subsequently, the solution stream is repressurized and rehomogenized by the rotating screws and carried foward through the feed opening into the extruder. The second section of devolatilization, situated physically behind the feed point, is commonly referred to as the rear vent section, whereas the following section, being the first vent downstream from the feed point, is referred to as the first vent section. It is anticipated that the devolatilization is associated with a process of bubble formation, growth and rupture.
With the extension of the above definition, the second vent downstream from the feed point is referred to the second vent section of devolatilization.
In general, in the vent sections, by means of reducing the diameter of the screw core, an extra free volume is created in the screw channel, that is to say, the screw channel is partially filled by the extruded materials, whereby the internal pressure in the screw channel is substantially reduced. In this way, the devolatilizing process can be carried out in each vent section in which the volatile materials aggregate and form bubbles and subsequently diffuse through the polymer fluids with the aid of agitation and escape to the surrounding atmosphere. Furthermore, if the vents are operated under vaccum, the higher efficiencies of removing volatile components will be obtained.
In order to further increase the efficiency of extrusion devolatilization performance, the accoustic treatments are introduced into the devolatilizing sections. The beneficial effects of using the accoustic treatments can be expressed in two ways; first, the ultrasonic waves propagating in the polymer fluid may induce profuse bubble formation immediately, and produce agitation to aid the aggregation of the volatile components to the bubbles, namely, the growth of the bubbles, thereby increasing the efficiency of removing the volatile components. Second, the ultrasonic waves produce a high-frequency stretch-compression stress field which is powerful to break bubbles floating on the surface of the polymer fluid, whereby in the srew channel the evaporation of volatile components remaining in the bubbles can be accelerated as well as a reduced internal pressure can also be mantained.
For the purpose of reducing the total volatile components to below 50.about.1000 ppm with a shorter processing time and permitting the plastic solution conveyed from the solution polymerization reactors to be worked continually by the extruders, it is thus desirable to introduce the accoustic treatments into extrusion devolatilization processes.